Microbial composition, structure, and mercury methylating activity are altered by nutrient exposure in periphyton in a contaminated watershed

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Alyssa Carrell, Dwayne A. Elias, Regina Wilpiszeski and Dawn Klingeman, Biosciences Division, Oak Ridge National Laboratory, Oak Ridge, TN, Grace Schwartz, Chemistry, Wofford College, Spartanburg, SC, Melissa Cregger, Department of Ecology & Evolutionary Biology, University of Tennessee, Knoxville, TN, Caitlin Gionfriddo, Smithsonian Environmental Research Center, Edgewater, MD, Ann Wymore and Scott Brooks, Oak Ridge National Laboratory, Oak Ridge, TN, Katherina Muller, Pacific Northwest National Laboratory, Richland, WA

Background/Question/Methods
The conversion of mercury (Hg) to monomethylmercury (MMHg) is a critical area of concern in global Hg cycling. Periphyton biofilms may harbor significant amounts of MMHg but little is known about the Hg-methylating potential of the periphyton microbiome. To characterize the structure and composition of archaea/bacteria, fungi, and Hg-methylating microorganisms in periphyton communities we used high-throughput amplicon sequencing of the 16S rRNA gene, ITS2 region, and Hg-methylation gene pair (hgcAB) in a contaminated watershed in East Tennessee (USA) in summer and autumn. Additionally, we examined how nutrient amendments (nitrate and/or phosphate) altered periphyton community structure and function.
Results/Conclusions
We found that bacterial/archaeal richness in experimental conditions decreased in summer and increased in autumn relative to control treatments, while fungal diversity generally increased in summer and decreased in autumn relative to control treatments. Interestingly, the Hg-methylating communities were dominated by Proteobacteria followed by Candidatus Atribacteria across both seasons. Surprisingly, Hg methylation potential correlated with numerous bacterial families that do not contain hgcAB, suggesting that syntrophic interactions within the periphyton communities influence rates of Hg transformation. To further explore these complex community interactions, we performed a microbial network analysis and found the nitrate amended treatment resulted in the highest number of hub taxa that also corresponded with enhanced Hg-methylation potential. This work provides insight into community interactions within the periphyton microbiome that may contribute to Hg cycling and will inform future research which will focus on establishing mixed microbial consortia to uncover mechanisms driving shifts in Hg cycling within periphyton habitats.